1. Field of the Invention
The present invention relates to a photoelectric converter that receives a light reflected from an original to which the light is irradiated, and converts the light into an electric signal, and more specifically to a linear image sensor suitable for an image reading device such as a facsimile or an image scanner.
2. Description of the Related Art
FIG. 16 shows a circuit diagram of an image sensor IC used in a conventional image reading device, and FIG. 17 shows a timing chart. (For example see JP 11-239245 A (pages 2 through 5, FIG. 1)).
An N-type region of a photodiode 101 is connected to a positive supply voltage terminal VDD, and a P-type region is connected to a drain of a reset switch 102 and a gate of a source follower amplifier 103. A source of the reset switch 102 is supplied with a reference voltage VREF1. A source of the output terminal of the source follower amplifier 103 is connected to a read switch 105 and a constant current source 104. A gate of the constant current source 104 is supplied with a constant voltage of a reference voltage VREFA. A photoelectric conversion block An shown in FIG. 16 shows a photoelectric conversion block of an n-th bit. The number of photoelectric conversion blocks is identical to the number of pixels, and the photoelectric conversion blocks are connected to a common signal line 106 through the respective read switches 105.
The common signal line 106 is inputted to an inverse terminal of an operational amplifier 109 through a resistor 110, and an output terminal of the operational amplifier 109 is connected to an output terminal 116 through a chip select switch 112 and a capacitor 113. The common signal line 106 is connected to a signal line reset switch 107, and a source of the signal line reset switch 107 is given a reference voltage VREF2. A resistor 111 is connected between the output terminal and the inverse terminal of the operational amplifier 109, and a non-inverse terminal of the operational amplifier 109 is fixed to a constant voltage VREF3. An inverse amplifier D is composed of the operational amplifier 109, the resistor 110 and the resistor 111.
An output terminal 116 of the image sensor is connected to a drain of the MOS transistor 114, and a source of the MOS transistor 114 is given a reference voltage VREF4. Further, the output terminal 116 of the image sensor is also connected to a capacitor 115 such as a parasitic capacitor. A clamp circuit C is composed of the capacitor 113, the capacitor 115 and the MOS transistor 114.
However, in the image sensor of the above type, the photodiode is reset after an optical signal is read subsequent to the completion of photocharge storage, and thereafter the reference signal is read, and a difference between the optical signal and the reference signal is taken. This leads to such a problem that reset noises put on the reference signal and the optical signal are different from each other. That is, because the reset noises of the different timings are compared with each other, there arises a problem in that the random noises are large.